1. Field of the Invention
The invention relates to a method for the manufacture of a solid and, more particularly, to a method for the manufacture of a solid that is practically insoluble in water and is non-pollutant.
2. Description of the Prior Art
Waste-water, namely industrial effluent or municipal waste-water, more commonly called liquid waste or sludge, is usually treated by various physical/chemical methods aimed in particular at precipitating the heavy metals that they contain in the form of hydrates of metal oxides having relatively little solubility.
The water from these hydrates is then separated, for example by filtering. A cake is thus obtained, containing about 60% of water. The water coming from the separation can be sent back into the environment.
The particles constituting this cake generally have a diameter of less than 200 microns and are not bound to one another. Thus, the cake has only low crush resistance and practically no cohesion. Once dumped, it may therefore break up, and a part of the heavy metals that it contains will be released by leaching in water.
In order to overcome these drawbacks, the cake is sometimes treated by means of the method described in the patent No. FR-A-2,644,358. There is then obtained a solid having good physical/chemical characteristics due to the presence of substantial quantities of silicates and aluminates.
However, the treatment described in this patent requires the mixing of the raw sludge with a source of lime and large quantities of silica and/or alumina.
Now, silica and/or alumina are materials that may be costly and are not always available at the place of treatment.
Furthermore the presence, in the dumps, of a non-negligible quantity of matter based on silica and/or alumina, which cannot be considered as wastes per se is a major drawback that has hitherto seemed to be inevitable.
The present invention is aimed at proposing a method for the manufacture of a solid that overcomes the above-mentioned drawbacks and makes it possible, in particular, to obtain, at low cost, a solid that is practically insoluble in water and is non-pollutant, without using a source of silica and/or alumina and in reducing, to the very minimum, the introduction into this solid of matter that cannot be considered to be waste.